In recent years, the lack of the radio spectrum resources is more serious with the rapid development of the radio communication. Linear modulation that has higher utilization efficiency is adopted in many radio communication systems in order to make use of the limited spectrum resources more efficiently. Since both the phase and amplitude of the linear modulation signal carry useful information, any nonlinear amplification of such signal will result in the increasing of the error rate and the interference between adjacent radio channels, which should be avoided when designing a radio communication system. It is necessary to amplify the linear modulation signal with high linearity in order to avoid those two harmful effects when designing radio communication system.
The conventional method for improving the linearity of the radio frequency (RF) amplifier is to set the output power of the amplifier much lower than 1 dB compression point, i.e., to use a small part of relatively linear input-output characteristics of the radio frequency power amplifier to achieve high-linearity amplification, which is called power backoff. However, there are obvious disadvantages in the backoff technique. First, the high linearity is achieved at the expense of power efficiency, which means that it does not make use of the most of the power output capacity of the amplifier, and therefore the power efficiency of the amplifier is reduced and the cost of the amplifier is increased; secondly, low power efficiency means that most power will be dissipated in the form of heat, which will be a heavy load to the ventilating and dispersing system of the whole base station; finally, the low efficiency power amplifier needs a power supply system of higher capacity, which is also an important factor leading to the increase of the cost of the transmitter. In order to improve the power efficiency of the amplifier and reduce the cost of the transmitter, the most common way is to set the power amplifier in the state of low linearity and high power efficiency, and then to improve the linearity of the amplifier from outside by certain measures and those measures are generalized as linearization techniques.
The conventional linearization techniques include the feedforward linearization technique and the feedback linearization technique. These two techniques are widely used in various radio communication systems. However, both of them have disadvantages: the main problems of the feedforward linearization technique include low efficiency, complex structure and high cost of the amplifier; the feedback linearization technique has the problem of loop stability or bandwidth limitation. Accordingly, neither of these two techniques can provide wideband amplification with high linearity at the power efficiency level required by the next generation mobile communication base station.
Another commonly used linearization technique is pre-distortion technique, which can achieve higher power efficiency than the feedforward technique. The difference between the pre-distortion technique and the feedforward technique is that the compensation is implemented before amplification, i.e., the input signal is pre-distorted in advance, so that the overall effects of the signal obtained by pre-distortion and nonlinear amplification is close to what obtained by linear amplification. There are three kinds of pre-distortion technologies, i.e. digital baseband pre-distortion, analog baseband or intermediate frequency pre-distortion, and RF pre-distortion. Among the three, digital baseband pre-distortion technique based on the adaptation of the digital signal processing is developing rapidly in recent years. In conventional pre-distortion technique, the pre-distortion compensation parameter calculated in advance is stored in a single dimensional pre-distortion look-up table, and then an address value is calculated on the basis of the magnitude of the current input signal, after locating the corresponding pre-distortion compensation parameter at the corresponding position of the pre-distortion look-up table, the input signal is corrected by an algorithm (e.g. adding or multiplying with the original signal). The way of determining the compensation parameter in the table is to have the nonlinear effect of the amplifier compensated exactly by the pre-distortion signal.
In order to make the compensation effect of the pre-distortion linearization track the changes of the amplifier characteristics due to the external factors, such as seasons, weather, environment and so forth, and the internal factors, such as the operation point of the amplifier, the shift of the amplifier characteristics along with the term of use, a feedback branch is usually required. A part of the output of the amplifier is coupled to this feedback branch as the feedback reference signal, and adjusts the pre-distortion parameters in the pre-distortion look-up table based on the difference between the input signal and the feedback signal. Since it is impossible for the amplifier characteristics to change markedly in a very short period of time, the adjustment is usually a nonreal-time process.
However, the linearization bandwidth and the linearization performance provided by the digital pre-distortion linearization solution are both limited without taking some special measures, because there are some memory effects in the amplifier. The existence of the memory effects greatly influences the performance of the pre-distortion, and the wider the bandwidth, the more serious the effects of the memory effects. From point of view of time domain, when the memory effect exists, the distortion characteristics are related not only to the current input but also the previous inputs of the amplifier; and from the point of view of frequency domain, the memory effect means that the amplitude and phase of the nonlinear distortion components of the amplifier vary with the change of the modulation frequency of the input signal, and such a variable distortion signal cannot be completely compensated by means of the pre-distortion signal having fixed amplitude and phase.
FIG. 1A-D shows nonlinear intermodulation distortion of the amplifier when memory effects exist and do not exist.
FIG. 1A shows the situation when no memory effect exists in the amplifier. New components will be generated by non-linearity of the amplifier after a two-tone input signal is amplified by the amplifier, wherein IM3L (lower sideband third-order nonlinear intermodulation distortion component) and IM3H (upper sideband third-order nonlinear intermodulation distortion component) are generated in the lower sideband and upper sideband of the two-tone signal respectively by the third-order nonlinearity of the amplifier, because there is no memory effect in the amplifier, the amplitude and phase of IM3L are equal to that of IM3H respectively. FIG. 1A only shows the amplitude components of the intermodulation distortion signals.
FIG. 1B shows the case when memory effect exists in the amplifier. The amplitude of IM3L is not equal to that of IM3H due to the memory effect of the amplifier. The asymmetry of the amplitudes in the intermodulation distortion components is usually caused by the electrical memory effect of the amplifier. Although only the amplitudes of IM3L and IM3H are shown in the drawings, it does not necessarily mean that their phases are equal. Actually, neither the amplitude nor the phase of IM3L is equal to that of IM3H under normal conditions.
FIG. 1C-D shows another case when the memory effect exists in the amplifier. Although the amplitudes of IM3L and IM3H are equal, as shown in FIG. 3C, their phases are not equal actually. The asymmetry of the phases in the intermodulation distortion components is usually caused by the thermal memory effect of the amplifier. Suppose a pre-distortion signal having equal magnitude and opposite phase with respect to IM3L, it can only compensate IM3L exactly, but it can not compensate IM3H, finally, the resulted signal is the vectorial resultant of this pre-distortion signal and IM3H. Apparently, the resultant signal is not zero when φ≠0, and may be no less than the amplitude of IM3H before being compensated (φ≧30°).
Because the amplitudes and the phases of the intermodulation distortion compensation components of IM3L and IM3H generated based on the in-band nonlinear distortion characteristic of the amplifier are in conformity with each other, the fact that the memory effect shown in FIG. 1B-D will cause IM3L and IM3H to be asymmetry, which will affect the pre-distortion linearization performance seriously.
The symmetrical compensation signals cannot compensate un-symmetrical IM3L and IM3H completely due to the memory effect, because the result of pre-distortion compensation is very sensitive to the match of amplitude and phase between the compensation signals and the distortion signals.
Since the digital pre-distortion linearization solution generally determines the compensation parameters according to the in-band nonlinear distortion characteristic, i.e., the conversion characteristic of amplitude modulation to amplitude modulation and amplitude modulation to phase modulation (hereafter called AM-AM & AM-PM characteristic of the amplifier), and this kind of characteristic can only describe the intermodulation distortion in which the upper sideband and the lower sideband are completely symmetrical, and thus the determined compensation parameters can only compensate the intermodulation distortion in which the upper sideband and the lower sideband are completely symmetrical. However, the memory effect will cause asymmetry between the upper sideband and lower sideband intermodulation distortion components, and it is obvious that symmetrical compensation signals cannot compensate un-symmetrical signals.
FIG. 2 is a schematic block diagram showing a typical narrowband digital pre-distortion system of the prior art. An input signal 101 is modulated as digital baseband signal via a baseband modulator 102, and the signal generates an address signal via an addressing circuit 104, while the address signal is in proportion to the amplitude of the input signal. A corresponding compensation parameter is searched from the corresponding unit of a compensation parameter look-up table 107, and the compensation parameter is multiplied by the original modulation signal via a complex number multiplier 106 to generate a corrected pre-distortion signal. The pre-distortion signal is converted to analog baseband via a digital/analog converter 110, this pre-distortion signal is modulated to radio frequency via a quadrature modulator (up converter) 112, then it is transmitted via a transmitting antenna 118 after amplified by a power amplifier 116. A part of the output power of the power amplifier 116 is coupled to a directional coupler 117, and then quadrature-demodulated and down converted to an analog baseband via a quadrature demodulator (down converter) 114, and the feedback signal is converted to a digital baseband via an analog/digital converter 111. In the digital baseband, the feedback signal is compared with the original signal delayed by a delay 103 in a signal comparator 109, and the resulted error signal is used to control a parameter updating unit 108 to generate a parameter updating signal for updating the nonlinear compensating parameters of the amplifier in the compensation parameter look-up table 107. The delay of the delay 103 is regulated by a delay regulator 105, which obtains a delay-regulating signal by comparing the original input signal delayed by the delay 103 with the feedback signal. The carrier frequency signal of the device is provided by a local oscillator 115 to the quadrature modulator 112 and the quadrature demodulator 114, and a phase regulator 113 is disposed between the local oscillator 115 and the quadrature demodulator 114, which is used to regulate the RF phase difference between the forward amplifying branch and the feedback branch, so as to keep the whole system stable. Because such a compensation device corrects the non-linearity based on the magnitudes of current signals and the AM-AM & AM-PM characteristics of the amplifier, without taking into consideration of the memory effects of the amplifier, hereby it can only compensate the memoryless nonlinear distortion near the predetermined frequency point, so that the linearization performance achieved by the device and the linearization bandwidth provided by the device are limited.
U.S. Pat. No. 6,356,146 discloses an improved solution for the narrowband digital pre-distortion compensation solution shown in FIG. 2, as shown in FIG. 3, it includes: (1) changing the multiplication of the complex gain correction into a filtering compensation of a finite impulse response (FIR) filter, the advantage of such compensation consists in that the case that compensation changing with frequency is taken into consideration; (2) the parameters of the FIR filter 206 are saved in a three-dimensional compensation parameter data structure 207. The data structure has three address retrieval entries, i.e. amplitude retrieval entry 204A, differential retrieval entry 204B and integral retrieval entry 204C, as shown in the drawing, the whole data structure is three-dimensional, the compensation coefficient can be obtained by addressing the three retrieval entries, which is related to the amplitude of the current input signal, the integral value of the input signal (indicating the effects of previous signals) and the differential value of the input value (indicating the bandwidth of the input signal). The compensation relates to not only the amplitude of the current signal, but also the previous input signals and the change rate of the input signals, with such improvement, hereby this device can compensate the nonlinear characteristics varying with the modulation frequency and the time changing nonlinear characteristics of the amplifier. The retrieval entries of the three-dimensional compensation parameter data structure 207 are not limited to the above-mentioned three ones, FIG. 3 shows another retrieval mode instead of the integral retrieval entry 204C, in which the change of the temperature of the amplifier is sensed by a temperature sensor 219, then the sensed signal is converted to the digital band via digital/analog converter 211, and the address value of the corresponding dimension is calculated by a address retrieval calculator 204D. The FIR filter 206, the three-dimensional compensation parameter data structure 207, the amplitude retrieval entry 204A, the differential retrieval entry 204B and the integral retrieval entry 204C are named distortion compensation signal processor 220. This compensation solution takes the AM-AM & AM-PM characteristic of the amplifier which varies with time and the modulation frequency of the input signal into consideration, thus its compensation effect is better than the narrowband digital pre-distortion compensation solution shown in FIG. 2, and also the linearization bandwidth provided is wider. However, this solution is still based on the in-band AM-AM & AM-PM characteristic of the amplifier, its pre-distortion signals of the upper sideband and the lower sideband are symmetrical in frequency spectrum, but the memory effect is an out-of-band distortion and will usually result in the intermodulation distortion, in which the upper sideband and the lower sideband are not symmetrical, thus this kind of solution can not compensate the memorial nonlinear characteristics of the amplifier.
Contrary to the pre-distortion technique, the linearization performance of the feedforward technique is not affected by the memory effect, mainly because the signal is compensated after being amplified by an amplifier. The advantages of feedforward technique in terms of linearization bandwidth and the linearization performance lie in that it is not sensitive to the memory effects. If the problem of being sensitive to the memory effects in predistortion technique can be resolved, the technique will reach or exceed the level of feedforward technique in terms of the linearization bandwidth and the linearization performance.